The present invention relates to a magnetic sensor of orthogonal flux-gate type which can detect the intensity and direction of a magnetic field and a manufacturing method for such a sensor.
Recently, there have been ever-increasing demands for small-size inexpensive magnetic sensors with high precision which can detect the intensity of a magnetic field as well as the direction of the magnetic field, and these sensors are used for detection sensors for magnetic markers on a road related to an automobile assist cruise and advanced high way system, magnetic sensors used for electronic compasses and navigation systems on vehicle equipment, measuring magnetic sensors for a magnetic field of live body such as the heart, and detection magnetic sensors for use in non-destructive inspection for steel, etc.
With respect to conventional magnetic sensors of this type, examples thereof include: Hall elements, MR elements, MI elements, superconducting quantum interference devices (SQUID), magnetic sensors of parallel or orthogonal flux-gate type, etc. Among these, the Hall elements are poor in sensitivity, and MR elements, MI elements, etc. are inferior in that since a single element cannot detect the direction of a magnetic field, a plurality of them need to be installed. In contrast, magnetic sensors of both parallel and orthogonal flux-gate types are able to detect the intensity and direction of a magnetic field even when installed as a single sensor. Moreover, these sensors are superior in the linear property, temperature characteristic and resolution of the detection output, and in particular, from the viewpoint of detection precision, attention has been focused on those of orthogonal flux-gate type because of their high precision.
FIG. 1A is an explanatory drawing that shows the principle of an element of orthogonal flux-gate type; FIG. 1B is an explanatory drawing that shows a magnetic flux formed in the core; FIG. 2 is an explanatory drawing that shows the operation thereof; and FIG. 3 are waveform diagrams showing an exciting current, a degree of magnetization in the core length direction and an output voltage of the detection coil, in the case when detection for a magnetic field is made by using the element shown in FIG. 1A.
Reference numeral 21 is a bar-like conductor made by a conductive material, 22 is a cylindrical core made by a soft magnetic material, 23 is a detection coil, and 25 is a high-frequency power source. The bar-like conductor 21 is placed coaxially with the core 22 through the inside of the core 22, and the bar-like conductor 21 is connected to the high-frequency power source 25. When the magnetic sensor of this type is placed with the axial line of the bar-like conductor 21 and the core 22 aligned in parallel with the direction of a magnetic field to be measured, the magnetic flux inside the magnetic field to be measured is attracted toward the core 22 side as illustrated in FIG. 2(a), so that a magnetic path is formed through the core 22.
When an exciting current IEX having a sine-wave as shown in FIG. 3 is flowed through the rod-like conductor 21, the peripheral face of the core 22 is magnetized as indicated by arrow in FIG. 2(b) so that the exciting current IEX is allowed to increase from a state shown in FIG. 3(a), and when it reaches a maximum value as shown in FIG. 3(b), the magnetization of the core 22 reaches a saturated state so that the magnetic flux of the magnetic field to be measured is separated from the core 22 and aligned in parallel with the bar-like conductor 21. In this state, the degree of magnetization of the core 22 in the length direction drops in a manner as shown in FIG. 3, and the output (voltage) of the detection coil 23 increases at a position where the rate of change in the magnetization in the length direction is great, and at a position where the rate of change in the exciting current IEX is great and when the current IEX reaches a maximum value or a minimum value, the output (voltage) of the detection coil 23 becomes zero.
During the state in which the exciting current IEX decreases from the maximum value and reaches the zero-crossing point, as shown in FIG. 2(c), the magnetic flux of the magnetic field to be measured is again allowed to pass through the core 22. When the direction of the exciting current IEx is reversed, the peripheral face of the core 22 is magnetized in a reverse direction to the circumferential direction as indicated by arrow in FIG. 2(d) and the exciting current IEx decreases to reach a minimum value, the magnetization of the core 22 is again allowed to reach a saturated state; thus, the magnetic flux of the magnetic field to be measured is aligned in parallel with the axial line of the core 22. During this state, the output of the detection coil 23 repeats changes in which it becomes greater in the area where the exciting current IEX is great while it becomes zero when the exciting current IEX reaches the minimum value, with the result that it has a change corresponding to 2 cycles in response to a change in the exciting current IEX corresponding to one cycle.
In other words, the exciting current is allowed to flow through the cylinder-shaped core 22 made of a soft magnetic material so as to excite it in the circumferential direction periodically so that the magnetization in the length direction of the core 22 is switched; thus, the relationship between the core 22 and the magnetic field to be measured is changed from FIG. 2(a) to FIG. 2(b), from FIG. 2(b) to FIG. 2(c) and from FIG. 2(c) to FIG. 2(d). In this state, the density of the magnetic flux, which resides around the detection coil 23, is allowed to change so that, as illustrated in FIG. 3, an output voltage (the phase of the output voltage) corresponding to the intensity (direction) of the magnetic field to be measured is obtained from the detection coil 23.
In an element of such an orthogonal flux-gate type, the flux distribution formed by the exciting current IEX flowing through the rod-like conductor 21 is shown by FIG. 1B. In other words, the magnetic fluxes are formed not only in the core 22 (indicated by a broken line s in the Figure), but also in the circumferential direction (indicated by a broken line t in the Figure) of a space outside the core 22. As a result, most of them only excite the space, and the magnetic flux fails to concentrate the magnetic field on the core 22, resulting in degradation in magnetic efficiency and wasteful consumption of the exciting current IEx. Moreover, in the element of orthogonal flux-gate type, since the detection coil 23 is an indispensable member, one portion of the magnetic fluxes (the broken line t in the Figure), generated in the space outside the core 22, come to reside around the detection coil 23, causing an exciting signal to be mixed with the detection output and resulting in degradation in the S/N ratio and resolution. Furthermore, the actual construction of the element of orthogonal flux-gate type is complex as compared with the Hall elements, MR elements, etc., although it is schematically shown in FIGS. 1A and 2 so as to show the principle thereof; therefore, another problem is that it is difficult to miniaturize the construction.
Here, Japanese Patent Application Laid-Open No. 10-90381(1998) has proposed a magnetic detection element shown in FIG. 4. FIG. 4 is a schematic view that shows the constriction of the conventional magnetic detection element disclosed by the above-mentioned patent application, in which a bar-like conductor 61 made of a copper wire is coated with an insulating layer 62, and inserted into a soft magnetic tube 63 coaxially, and one end of the bar-like conductor 61 is connected to a ground conductor 65 through a conductor 64. In such a conventional magnetic detection element, a great change in impedance is generated by utilizing a frequency in the vicinity of a resonant point generated by an inductance L and a stray capacitance C by the soft magnetic tube 63, and this change is taken so as to detect the magnetic field.
In the conventional magnetic detection element of this type, the ground conductor 65 forms a LC resonant circuit with the stray capacitance, and is placed in the proximity of the bar-like conductor 61 that forms an inductance so as to eliminate a change in the stray capacitance due to environments. Since this magnetic detection element requires no detection coil, it is originally not necessary to take into consideration the influences from the magnetic fluxes residing around the detection coil. Therefore, it is not necessary to prevent an outward expansion of exciting magnetic fluxes generated from the magnetic detection element.
Here, since this element is a two-terminal element for detecting a change in the impedance with respect to the outside magnetic field, it only responds to the intensity of a magnetic field, and does not respond to the direction of a magnetic field, thereby failing to detect the direction of a magnetic field. Moreover, the magnetic detection element having the above-mentioned construction is characterized in that the change in impedance is detected; therefore, since no magnetic switching is required, the value of a current flowing through the conductor is small, and it is not necessary to take into consideration an increase in power consumption caused by excitation of the space in practical use. Consequently, it is not possible to apply a construction including the above-mentioned magnetic detection element to the element of orthogonal flux-gate type.
An object of the present invention is to provide a magnetic sensor of orthogonal flux-gate type which can achieve a small size and light weight by using a simplified construction, without losing its inherent characteristics as an element of orthogonal flux-gate type, and a manufacturing method of such a magnetic sensor.
Another object of the present invention is to provide a magnetic sensor of orthogonal flux-gate type which can achieve a great reduction in power consumption and provide a high sensitivity, without losing its inherent characteristics as an element of orthogonal flux-gate type, and a manufacturing method of such a magnetic sensor.
The magnetic sensor of the present invention has an arrangement in which a high-frequency current is allowed to flow through an internal conductor that is placed inside a cylindrical core made of a soft magnetic material so that a magnetic field to be measured residing around a detection coil wound up on the core is changed, and the intensity and direction of the magnetic field to be measured is detected on the basis of an output of the detection coil. In this arrangement, the magnetic sensor is further provided with an external conductor placed around of the core, which is electrically connected to the internal conductor.
Since the external conductor is placed around the core, no space is magnetized by the magnetic field generated by the current flowing through the internal conductor so that the magnetic field can be concentrated on the core; thus, it is possible to improve the magnetic efficiency, and consequently to cut the exciting current. Moreover, since the magnetic field, formed by the flow of the exciting current, can be concentrated on the core, it is possible to efficiently excite the core even by the use of low power, and also to improve the S/N ratio by preventing an exciting signal from being mixed with the output signal of the detection coil. Furthermore, since the entire core, that is, not only the surface thereof, but also the inside thereof, can be magnetically excited in a uniform manner, it is possible to eliminate the generation of a residual magnetic field and the resulting hysteresis and consequently to obtain high measuring precision.
In the above-mentioned magnetic sensor, the external conductor is placed at least two positions that faces each other with the core located in between. Since the side faces at two corresponding positions of the core are covered with the external conductor, the sensor has a simplified structure, can be easily manufactured in a small size, and can correctly detect the intensity and direction of a magnetic field.
In the above-mentioned magnetic sensor, the external conductor has an arrangement in which two ends of a channel member are closed with end plates having holes for allowing the internal conductor to pass through them. Thus, since the external conductor is formed by the channel member, the structure is simplified and it becomes possible to easily manufacture the sensor.
In the above-mentioned magnetic sensor, the external conductor is formed into a cylindrical shape. Since the external conductor is formed into the cylindrical shape, the entire periphery of the core can be covered with the external conductor; thus, it is possible to easily magnetize the core, to reduce the power consumption, and also to provide a structure with high rigidity that is less susceptible to impact, vibration, etc., as well as a high production efficiency in mass production.
In the above-mentioned magnetic sensor, the internal conductor is formed into a column shape, and both the external conductor and core are formed into a cylindrical shape. The magnetic field, which is formed by magnetizing the core by the exciting current, is not allowed to expand in space, and concentrated on the core; therefore, it is possible to efficiently excite the core even by the use of low power, and also to improve the S/N ratio by preventing an exciting signal from being mixed with the output signal of the detection coil. Furthermore, since the entire core, that is, not only the surface thereof, but also the inside thereof, can be magnetically excited in a uniform manner, it is possible to eliminate the generation of a residual magnetic field and the resulting hysteresis, and consequently to obtain high measuring precision.
In the above-mentioned magnetic sensor, one or a plurality of slits, extending in the axis length direction, are formed in the peripheral wall of the external conductor. Since the slits are formed in the external conductor, it is possible to prevent an eddy current from being generated in the external conductor, and consequently to prevent a reduction in the output.
In the above-mentioned magnetic sensor, the internal conductor and the external conductor are integrally connected with the respective ends on the same side being electrically connected. Since the respective ends on the same side of the internal conductor and external conductor are integrally connected, it is possible to obtain high rigidity in structure, and consequently to obtain stable measuring precision.
In the above-mentioned magnetic sensor, the core is formed by a tube made of permalloy or sendust. Therefore, a cylindrical core having a small diameter with high precision can be manufactured at low costs with high productivity.
The method of manufacturing the magnetic sensor of the present invention is provided with steps of: obtaining a cylindrical core made of a soft magnetic material; inserting and securing an internal conductor into the core with an insulating coat film interpolated between these; externally fitting and securing a cylindrical external conductor having slits in its axis length direction onto the periphery of the core with an insulating material interpolated between these; and winding a detection coil on the periphery of the external conductor. In the manufacturing method of the present invention, it is possible to mass product magnetic sensors with high precision at low costs with uniform quality without irregularities in precision.
In the above-mentioned manufacturing method of the magnetic sensor, the cylindrical core made of a soft magnetic material is formed by a tube made of permalloy. Thus, it is possible to obtain a cylindrical core having a great permeability even with a small coercive force. Moreover, it becomes possible to mass manufacture products with stable precision.
In the above-mentioned manufacturing method of the magnetic sensor, the tube made of permalloy is subjected to a heating treatment for several hours in a range of 1000xc2x0 C. to 1200xc2x0 C. so as to improve its soft magnetic property. Thus, it is possible to eliminate machining distortion, and consequently to obtain a tube having a preferable soft magnetic property. Since the uniform soft magnetic property is obtained, it is possible to reduce irregularities in precision, and also to greatly improve the resolution.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.